1. Field of the Invention
The present invention relates generally to methods of suppressing deposition of a radioactive isotope, and more particularly to a method of suppressing deposition of a radioactive isotope, which is suitable for application to formation of a ferrite film on a constituent element surface of a boiling-water reactor power plant.
2. Description of the Related Art
Boiling-water reactor (BWR) power plants, for example, are constructed so that fuel-generated heat is efficiently moved to cooling water by forcibly circulating via a recirculating pump or an internal pump the cooling water to a reactor pressure vessel that contains fuel rods. A large portion of the steam which has thus been generated by boiling of the cooling water in the reactor is used to drive a steam turbine to which a power generator is coupled. The steam, after being discharged from the steam turbine, is condensed by a condenser. The condensate that has been obtained by the condensation of the steam is almost completely deaerated within the condenser and supplied as cooling water for the reactor once again. At this time, the oxygen and hydrogen that have resulted from the radiolysis of water inside the reactor are also almost completely removed from the condensate within the condenser. The condensate, before being returned to the reactor, is cleared mainly of metallic impurities by a desalter and/or other ion-exchange resin filtering devices in order to suppress the occurrence of radioactive corrosion products in the reactor. After that, the condensate is heated to approximately 200 degrees C. and supplied to the reactor.
Radioactive corrosion products also occur in the pressure vessel and other sections brought into contact with the reactor water, such as a PLR (Primary Loop of Recirculation) system. Steel essentially free from corrosion, such as stainless steel or a nickel-based alloy, is therefore used to form the main constituent members of a primary system. Also, reactor pressure vessels made of low-alloy steel have the respective inner surfaces padded with stainless steel to prevent the low-alloy steel from coming into direct contact with the reactor water. In addition to these considerations associated with materials, a reactor water clean-up system is used to clean a portion of the reactor water and actively remove the metallic impurities slightly generated in the water.
However, even with the above-discussed anti-corrosion measures, the presence of very small quantities of metallic impurities in the reactor water are unavoidable, so some of the metallic impurities become deposited as metal oxides on the surfaces of the fuel rods. When irradiated with the neutrons emitted from the nuclear fuel inside the fuel rods, the metals that have deposited on the fuel rod surfaces cause a nuclear reaction and generate cobalt-60, cobalt-58, chromium-51, manganese-54, or other radioactive isotopes. Most of the radioactive isotopes remain deposited in the form of oxides on the fuel rod surfaces. Some of the radioactive isotopes, however, are eluted into the cooling water, or re-released as insoluble solid precipitates into the reactor water, in accordance with the solubility of the oxide within the isotope. The radioactive substances in the reactor water are removed by the reactor water clean-up system. While being circulated with the reactor water through the PLR system, unremoved radioactive substances accumulate on the surfaces of the constituent members. As a result, radiation is emitted from the surfaces of the constituent members, thus causing radiation exposure of the persons engaged in routine inspection. The inspection staff's radiation doses are controlled so as not to exceed defined values for each person. Under the situation of these defined values being lowered in recent years, the necessity is arising for reduction of each person's radiation dose to a level as low as economically achievable.
Accordingly, methods of reducing the deposition of radioactive isotopes on pipelines, and methods of reducing the density of the radioactive isotope in reactor water are being studied in various terms. JP-A-58-79196, for example, proposes a method of suppressing the ingress of a radioactive isotope such as cobalt-60 or cobalt-58 into an oxide film by injecting zinc or other metal ions into the reactor water and forming a zinc-containing fine oxide film on the surface of the PLR system piping brought into contact with the reactor water. Also, JP-A-62-95498, for example, proposes a method in which, before the radioactive isotope is eluted or released into the cooling water, an oxide film is formed under fixed conditions beforehand on the inner surfaces of the PLR system and reactor water clean-up system pipelines into which the reactor water flows during operation.
For the method of injecting zinc or other metal ions into the reactor water, which is described in JP-A-58-79196, depleted high-valence zinc ions must be continuously injected during operation to avoid radioactivation of the zinc itself. Also, for the method of forming an oxide film, which is described in JP-A-62-95498, there is a need to form the oxide film in, for example, a range of the operating temperature of 250-300 degrees C. of the BWR power plant.
Accordingly, a technique for forming a fine ferrite film on a material surface at a low temperature equal to or less than 100 degrees C. has been studied as a less expensive method of reducing the deposition rate of a radioactive isotope. Experiments with test pieces having a ferrite film formed on a metal surface have been conducted to find that, as shown in FIG. 2, the deposition of a radioactive isotope under a reactor water environment can be suppressed significantly.